Solid-state image sensors have found widespread use in camera systems. The solid-state imager sensors in some camera systems are composed of a matrix of photosensitive elements in series with switching and amplifying elements. The photosensitive sensitive elements may be, for example, photoreceptors, photo-diodes, phototransistors, charge-coupled device (CCD) gate, or alike. Each photosensitive element receives an image of a portion of a scene being imaged. A photosensitive element along with its accompanying electronics is called a picture element or pixel. The image obtaining photosensitive elements produce an electrical signal indicative of the light intensity of the image. The electrical signal of a photosensitive element is typically a current, which is proportional to the amount of electromagnetic radiation (light) falling onto that photosensitive element.
Of the image sensors implemented in a complementary metal-oxide-semiconductor (CMOS)- or MOS-technology, image sensors with passive pixels and image sensors with active pixels are distinguished. The difference between these two types of pixel structures is that an active pixel amplifies the charge that is collect on its photosensitive element. A passive pixel does not perform signal amplification and requires a charge sensitive amplifier that is not integrated in the pixel.
FIG. 1 illustrates bus driving in a conventional CMOS image sensor. In the conventional CMOS image sensor of FIG. 1, different columns are connected via a switch to a common bus that brings the signal to an output amplifier. The more detailed illustration of the connection topology is presented in FIG. 2A.
In FIG. 2, Rload and Cload are the parasitic bus metal resistance and capacitance, respectively. The bus driving speed is determined by the amplifier output impedance, column driving switch, the metal resistance, the metal capacitance and all the capacitances of the switches connected to this bus. In most cases, the switch is the limiting factor regarding bus driving speed because the switch cannot be made too large as this increases total bus capacitance. Therefore, the switch impedance is quite high which, in most cases, limits the bus driving speed. This reduces readout speed significantly and requires a lot of buses in a high speed sensor to get a high frame rate. This makes post-bus multiplexing more complex and increases die area which increases the total cost of manufacturing the image sensor.